One of the best solutions to power a laptop is to use one of the Carnetix P/S. It will handle all of the startup, shutdown proceedures as well as provide amp trigger, 12v for the screen and 5v for powered hubs.

Take a look at post #7 & #8 in my worklog below. I belive you will have a similar issue with the dell (as it needs an ID signal as well) but easily worked around.

I believe there another thread from about the same time I posted #7 & #8 that helped me but it was more for the Dell laptops.